MATERIALS AND METHODS - Environmental study in Cabras lagoon: thetrophic state and the microa

The study was carried out on 15 stations, located in the middle-terminal reaches of four natural water courses and four canals (Table I). The collection of water and epilithic diatoms samples was effectuated in different periods between April 2008 and April 2009.

In all the stations the most significant physical-chemical and microbiological parameters were measured. Temperature, pH, conductivity and dissolved oxygen were measured in situ with a CTD probe (YSI556), whereas alkalinity, Cl^-, BOD5, soluble reactive phosphorus (SRP) and total phosphorus (TP), N-NH3, N-NO3, N-NO2, total nitrogen (TN), reactive silica (RSi) and total suspended solids were determined in laboratory, according to Italian Standard methods (IRSA 1994). The microbiological analysis considered the research of Escherichia coli, faecal and total coliforms and was effectuated using the method of filtration on membrane (IRSA 1994).

Diatom samples were collected scraping the substrate, principally rocks and stones, with a small hard brush and always in full current (APAT 2005). The 21 samples totally collected were immediately fixed in situ with 4% neutralized formaldehyde. Contrary to other systems, in Rio Mannu of PortoTorres, subjected to heavy anthropic pressures, diatoms

Silvia Pulina, Environmental study in Cabras Lagoon: the trophic state and the microalgal component with particular attention to Harmful Algal Species, PhD School in Natural Sciences, University of Sassari. 152 were collected in two samplings, respectively in winter and spring. The aim was to follow the temporal evolution of the biological water quality. The winter sampling, started in December 2008 was completed in March 2009 because of the notable flow due to elevated rainfall in three of the six stations. Diatom samples were treated in the laboratory with hydrogen peroxide (H2O2) and acetic acid (CH3COOH) under heat, until the complete oxidation of the organic substance (Schrader 1973). The cleaned frustules were mounted on permanent slides using Storax resin (index of refraction 1.59). The taxonomic determination at a specific and infraspecific level was carried out using an inverted optical microscope (Zeiss, Axiovert 10) at 1000 magnification and using monographs by various authors: Bourrelly (1981), Germain (1981), Hustedt (1985), Krammer & Lange-Bertalot (1986, 1988, 1991), Lange-Bertalot et al. (2003). To uniform the procedure with already adopted by other European countries, the valuation of the abundance of the species was done with the count of about 400 valves and/or frustules (Ciutti et al. 2004). To calculate the indices EPI-D and IBD the manuals by Dell’Uomo (2004) and Prygiel & Coste (2000) were used. The results of EPI-D were converted in scale from 1 to 20 and compared with those obtained by the application of the IBD method.

RESULTS

The diatom flora of water courses

The analysis of epilithic diatoms has revealed a total presence of 152 taxa belonging to 34 genera (Table II). The diatom flora observed is nevertheless made up of other species, in some cases rare, in others infrequent and abundant, not yet identified with certainty.These taxa, even if they are not determining in the valuation of the biological water quality are of relevant interest for the biodiversity because they are part of the periphytic communities of the water courses investigated. Among the detected genera, four belong to order of Centrales and 30 to Pennales and the major number of species belong to genera Navicula (41), Nitzschia (21), Fragilaria (12), Achnanthes (11) and Gomphonema (10). The total number of taxa for each sample varied from a minimum of 12 in the station 4A to a maximum of 48 in the station 2D. The major part of the observed diatoms was common in several stations but some taxa resulted exclusive for only one of them. Generally, the species found are considered cosmopolitan and typical of environments with mediocre and bad quality. Among the taxa most frequent and sometimes abundant in the samples analized, Cocconeis placentula var. pseudolineata Geitler and Navicula confervacea (Kützing) Grunow in Van Heurk do not result in the list of the method EPI-D. N.

confervacea (Fig. 2), an invasive species (Coste & Ector 2000), already observed in the north of the island (Lai et al. 2007), is also present in the Tirso River and in the canals of Mare Foghe system (central Sardinia). This thermophile species appeared in the spring and summer samples with a range of temperature of 11.9-22.6 °C. The presence of Achnanthes rupestoides Hohn (Fig. 3), another tropical taxa until now never seen in Sardinia, was highlighted. This species was present in the two stations upstream of the Rio Mascari tributary, one of the most important affluents of Rio Mannu in Porto Torres. The diatom assemblages in the canals of Mare Foghe catchment, have shown differences in the floristic composition, in respect to those of other investigated systems, with species like Pleurosira laevis (Ehrenberg) Compère, Thalassiosira weissflogii (Grunow) Fryx. and Hasle, Achnanthes inflata (Kützing) Grunow, Achnanthes brevipes Agardh, Cymatopleura solea var.apiculata (W.Smith) Ralfs, Neidium dubium (Ehrenberg) Cleve, never abundant in these stations and absent in all the other water courses investigated.

The quality of the water courses

Silvia Pulina, Environmental study in Cabras Lagoon: the trophic state and the microalgal component with particular attention to Harmful Algal Species, PhD School in Natural Sciences, University of Sassari. 153 The values of the physical-chemical and microbiological parameters are reported in Table III. Tables IV and V reports the results of the diatom indices and the range of EPI-D and

Table II. Genera and diatom taxa identified.

IBD with the relative classes and levels of quality. Results of diatom indices are reported in the Fig. 4.

As regards the system of Rio Mannu of Porto Torres, all the investigated stations resulted with phenomena of water contamination, with significant values of algal nutrients and microbiological parameters. The diatom indices demonstrated that the most contaminated stations were the 1 and 2, those more upstream on the affluent of Rio Mascari, and station 6 on Rio Mascari, that receives urban and industrial wastes. Both the indices also indicated a decline in the quality of the waters during the spring period, coinciding with an important reduction of flows. Moreover, in the same period, a reduction of the number of taxa was observed in stations 1, 3 and 6. This sign of a probable simplification of the diatom

Silvia Pulina, Environmental study in Cabras Lagoon: the trophic state and the microalgal component with particular attention to Harmful Algal Species, PhD School in Natural Sciences, University of Sassari. 154 assemblages corresponded with a decline of quality. Seasonal variation of biological quality in Mannu of Porto Torres system are reported in Fig. 5.

Table III. Results of physical, chemical and microbiological variables analysed. Results of Mannu of Porto Torres (1A-6A) system are mean values of different samplings.

Parameters Stations of water courses

Catchments Water courses Stations Date of sampling EPI-D IBD

1A 02/12/2008 - 07/04/2009 6.9 6.9

Table V. Ranges of values EPI-D and IBD, classes and water quality assessment.

EPI-D IBD Quality Class Color

Fig. 4 EPI-D and IBD results in the different stations of the four systems (A = Mannu of Porto Torres; B = Padrongianu; C = Mare Foghe; D = Tirso).

Data of Mannu of Porto Torres system are mean values of winter and spring samplings.

Fig. 5 Seasonal variation of EPI-D and IBD in Mannu of Porto Torres system.

For Padrongianu River the diatom indices confirmed a decline of the water quality, from upstream to downstream, highlighted also by the physical-chemical and microbiological parameters. In the highest station both indexes indicated a situation of good quality. For station 2, located in correspondence with the waste discharge of the town of Olbia, the quality in relation to EPI-D and IBD was respectively mediocre and good. The last station lowest down resulted the most polluted with a mediocre-bad quality for EPI-D and bad for IBD. The dominant species was N. incospicua a good indicator of meso-eutrophic environments (Dell’Uomo 2004).

In the catchment of Rio Mare Foghe, Rio Iscas and Rio Mannu were the worst in terms of quality. Both have demonstrated the highest values levels of BOD5, N-NO2, SRP, and TP.

The microbiological investigation also showed a contamination of organic nature. For both water courses the EPI-D and IBD indices indicated, in agreement, a state of bad quality.

The dominant species were Nitzschia incospicua Grunow, Cyclotella meneghiniana Kützing and Bacillaria paxillifer (Muller) Hendey. The last two taxa, in particular, are considered excellent indicators of eutrophic environments (Dell’Uomo 2004). EPI-D and IBD also indicated a mediocre-bad and bad quality for Rio Tanui, which showed higher values of nutrients and microbiological parameters in respect to Rio Pauli Gippa, its affluent. In the latter both indexes indicated a mediocre quality.

Finally, for the two stations on Tirso River, the quality was mediocre and both indexes showed a slight improvement of the biological quality of the water in station 2, downstream from the industrial area of Ottana. In this station, higher algal nutrients and lower values for the microbiological parameters were observed, too.

DISCUSSION

The study carried out offers a contribution to knowledge of diatom flora in Sardinia water courses and, at a more general level, in the Mediterranean area. The previous list of 157 taxa already obteneid with others investigations (Lai et al. 2007) is extended with other 50 taxa. In particular, the presence of A. rupestoides, that like N. confervacea is considered an invasive species, is noted. The observation of these tropical species maybe an important signal of the possible heating of continental freshwaters. More studies will better define the entity of the distribution of these species in waters of the island. Such considerations are true for each of the elements of the Sardinian diatom flora. To reach this result, it is necessary the knowledge of the specific composition, the distribution and the ecological

Silvia Pulina, Environmental study in Cabras Lagoon: the trophic state and the microalgal component with particular attention to Harmful Algal Species, PhD School in Natural Sciences, University of Sassari. 157 needs of the taxa on different spatial and temporal scales. The gathered informations have interested until now high anthropic pressure water courses, whereas there are not indications about assemblages present in modest or absent impact conditions. For this, it is important to consider a wider number of stations along the water courses, to understand and value the dynamics on the entire considered basins. A further objective is the integration of the list of taxa actually considered by the EPI-D method, with the relevant species in the Sardinian context, to contribute to a better application of the index in the island and in general in the Mediterranean islands. The majority of the observed species is common to those indicated in the water courses in Sicily and Corse (Finocchiaro & Ferlito 2007), (Rimet et al. 2005 b). The lay-out of a first check-list of the observed species could result particularly useful to help future studies.

This investigation permits to confirm the utility of diatom indices also in Sardinia for the valuation of the quality of lotic waters and underlines their utility also for the management of the territory. The EPI-D and IBD methods have consented to more complete and coherent valuation in the situation of the territory, and the impacts present in respect to those given by only the traditional parameters. The results obtained were quite agreeable with those of physical-chemical and microbiological parameters and both indexes, even if with slightly different values, showed a good applicability in all lotic contexts investigated.

However EPI-D values obtaneid seem more reliable. The systems of Rio Mannu of Porto Torres and of Rio Mare Foghe, resulted those with stronger phenomena of water contamination in all the stations examined. The typology of the pollution observed seemed to correspond to that of organic nature, for the most part due to agricultural and zootechnical activities carried out in the basins and to civil wastes not adequately depurated.

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